Downhole Actuating Apparatus

ABSTRACT

A mechanical counting device for actuating a plurality of output devices, the device comprising: linear indexing means adapted to count a plurality of actuating signals and to cause actuation of the output devices when a predetermined number of actuating signals for each output device has been received, wherein the mechanical counting device is adapted to cause actuation of a particular output device when a different predetermined number of actuating signals has been received such that the output devices are sequentially actuable.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT/GB2011/050467, filed Mar. 10,2011, which claims priority to United Kingdom Patent Application No.GB1005133.2, filed on Mar. 26, 2010, the contents of each oneincorporated herein by reference.

FIELD OF INVENTION

The present invention relates to mechanical devices for counting inputsignals. In particular, the invention relates to mechanical devices forcounting input signals to actuate downhole tools in a sequential manner.

BACKGROUND TO INVENTION

There are many situations in which downhole tools must be selectivelyactuated. However, communicating with the tools to cause actuation canbe difficult in the downhole environment. Systems such as RFID systemsexist but these are complex, expensive and prone to failure. Indeed, anyform of electrical, electronic or magnetic device is often not robustenough to withstand the harsh downhole environment.

During hydraulic fracturing of a multiple zone well, a series of tools,or clusters of tools, are provided at each zone, and each downhole toolneeds to be actuated and fluid is diverted to flow outwards to fracturethe well. The actuation must be performed in a sequential manner toallow the borehole to be progressively fractured along the length of thebore, without leaking fracture fluid out through previously fracturedregions.

Due to the expense and frequent failure of electronic or electricaldevices, the most common approach to tool actuation is still fullymechanical. Balls of ever increasing size are dropped down a tubularpositioned within the well bore. The tools are configured so that thefirst dropped ball, which has the smallest diameter, passes though thefirst and intermediate tools, which have a ball seat (hereinafterreferred to as a valve seat) larger than the ball, until it reaches thefurthest away tool in the well. This furthest away tool is configured tohave a valve seat smaller than the first dropped ball so that the ballseats at the tool to block the main passage and cause transverse portsto open thus diverting the fluid flow. Subsequently dropped balls are ofincreasing size so that they too pass through the nearest tools but seatat further away tools which have a suitably sized valve seat. This iscontinued until all the tools have been actuated in the order offurthest away to nearest.

Therefore, this approach does not involve counting the dropped balls.Balls which are too small for a particular tool are simply notregistered. However, this approach has a number of disadvantages. Thenumber of tools with varying valve seats that can be used is limited inpractice because there must be a significant difference in the size ofthe seat (and therefore the ball) so that the ball does notinadvertently actuate previous tools. Also, the valve seats act asrestrictions to flow through the tubular which are always undesirable.The smaller the seat the greater the restriction.

It is desirable to provide an apparatus which allows: actuation of alarge number of downhole tools; and/or downhole tools with the same sizeof valve seat; and/or valve seats with the largest possible diameter.

SUMMARY OF INVENTION

According to a first aspect of the present invention there is provided amechanical counting device locatable at each of a plurality of downholetools arranged within and along a well bore, each tool having a mainbore corresponding to the tubular positioned in the well bore, and eachtool being actuatable to open one or more fluid ports which aretransverse to the main bore, the mechanical counting device comprising:

linear indexing means adapted to cause the mechanical counting device tolinearly progress along the main bore by a predetermined distance inresponse to receiving an actuating signal until reaching an actuationsite of the tool whereupon the tool is actuated,

wherein the mechanical counting device is locatable at a plurality ofdifferent predetermined positions within the main bore such that thedownhole tools are sequentially actuatable.

The mechanical counting device may be adapted to engage with one of aplurality of longitudinal recesses provided along the main bore.

The mechanical counting device may be adapted to linearly progress alongthe main bore by the predetermined distance in response to an object,such as a ball, dropped within the tubular positioned within the wellbore, which thus provides the actuating signal.

The mechanical counting device may be adapted, upon reaching theactuation site, to cause the dropped object to stop at the tool, thusblocking the main bore at the tool.

The mechanical counting device may be adapted to linearly progress in anumber of discrete steps to the actuation site. Each discrete step maycorrespond to the mechanical counting device moving from onelongitudinal recess to the adjacent longitudinal recess.

The mechanical counting device may comprise a collet member having anumber of fingers and a protrusion provided at the end of each finger.Each finger may be flexible. The collet member may comprise a tubularmember having a bore which is sized such that the dropped object maypass through the tubular member. Each finger may be movable between afirst position in which the protrusion is outwith the bore of thetubular member and a second position in which the protrusion is withinthe bore of the tubular member and contactable by the dropped object.Each finger may be bendable between the first and second positions.

The collet member may be locatable within the main bore such that theprotrusion of one or more fingers is engaged with a recess when thefinger is at the first position and not engaged with a recess when thefinger is at the second position.

The collet member may comprise a first set of fingers and a second setof fingers which is longitudinally spaced from the first set. The colletmember and the recesses may be configured such that, when the fingers ofthe first set are engaged with a recess, the fingers of the second setare not engaged with a recess. The collet member and the recesses may beconfigured such that, when the fingers of the second set are engagedwith a recess, the fingers of the first set are not engaged with arecess.

The collet member may be adapted such that the dropped object passingthrough the main bore contacts the protrusion of the one or more fingerswhich are at the second position such that the collet member is linearlymoved in the direction of travel of the dropped object. The colletmember may be linearly moved until the protrusion engages with the nextrecess. The collet member may be adapted such that engagement with thenext recess allows the dropped object to continue past the set offingers of which the protrusion has engaged with the next recess.

The collet member may be adapted such that the linear movement causesthe protrusion of the one or more fingers which are at the firstposition to disengage from the recess and move to the second position.The collet member may be linearly moved by the impact force from thedropped object and/or by fluid pressure upstream of, and acting on, thedropped object.

In this manner, the collet member is linearly movable in a stepwisesequence, moving one recess every time an object is dropped.

The mechanical counting device may be movable towards a sleeve memberprovided within the main bore and adapted to block the transverse ports.The collet member may be adapted to contact and act upon the sleevemember upon reaching the actuation site to move the sleeve member andcause fluid communication between the main bore and the transverseports.

In this manner, the collet member is linearly movable one recess at atime towards the actuation site whereupon it causes moving of the sleevemember to open the transverse ports. The main bore of each tool can beprovided with a large number of recesses. For a particular tool, thecollet member can be located a particular number of recesses from theactuation site. The number of recesses can be arranged to vary for eachtool depending on its proximity to the surface. For instance, the toolfurthest from the surface could have the least number of recesses, suchas only one, while the tool nearest the surface could have the greatestnumber of recesses, such as fifty if there is a total of fifty toolswithin the well bore. The tools will therefore sequentially actuate inthe order of furthest away to nearest.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 is a (a) perspective view and a (b) sectional side view of ahousing of a tool (shown in FIG. 3) of a downhole actuating apparatus;

FIG. 2 is a (a) perspective view and a (b) sectional side view of acollet of a downhole actuating apparatus;

FIG. 3 is a sectional side view of a tool of a downhole actuatingapparatus with a sleeve in the closed position;

FIG. 4 is a detailed sectional side view of a portion of the tool ofFIG. 1 with a ball approaching the tool;

FIG. 5 is a detailed sectional side view of a portion of the tool ofFIG. 1 with the ball landing at the first seat;

FIG. 6 is a detailed sectional side view of a portion of the tool ofFIG. 1 with the ball landing at the second seat;

FIG. 7 is a detailed sectional side view of a portion of the tool ofFIG. 1 with the ball released; and

FIG. 8 is a (a) perspective view and a (b) sectional side view of a dogassembly.

DETAILED DESCRIPTION OF DRAWINGS

FIG. 1 shows a downhole tool 10 of a downhole actuating apparatus. Theapparatus comprises many of these downhole tools 10, such as fifty,which can be secured to a tubular and sequentially arranged along a wellbore. As utilized throughout this specification, the term “tubular”refers to any generally tubular conduit for transporting fluid,particularly oil, gas and/or water, in and/or from a subterranean well.A “tubular” as deployed in a subterranean well, may be formed fromindividual, discrete lengths of generally tubular conduit usuallysecured together by means of collars to form, for example a tubingstring, drill string, casing string, liner, etc., which is positioned ina subterranean well and utilized, at least in part, to transport fluids.The tubular may have a bore of a generally uniform diameter throughoutthe length thereof or may have two or more sections having bores ofdifferent diameters. For example, the tubular may be comprised of acasing string positioned within the well bore, extending at one endthereof from the well head, either surface or subsea, and connected ator near the other end thereof to a tubing string or liner having a borethat is smaller than that through the casing string. As another example,the tubular may be comprised of a tubing string positioned within thewell bore, extending at one end thereof from the well head, eithersurface or subsea, and connected at or near the other end thereof to acasing string or liner having a bore that is larger than that throughthe tubing string. Environments other than a subterranean well in whichtubulars may be used in accordance with the present invention, include,but are not limited to, pipelines and sewer lines.

In this embodiment, the tools 10 are provided for the purpose of wellfracturing. Each tool 10 has a main bore 12 which in use is coaxial withthe tubular positioned within a well bore and a number of transversefluid ports 14. The main bore 12 of the tool 10 defines a number ofannular grooves or recesses 16, the recesses 16 each being equally andlongitudinally spaced apart by a predetermined spacing. The number ofrecesses 16 can be configured to be the same as the total number oftools 10.

Inserted within the main bore 12 of each tool 10 is a collet 20 as shownin FIGS. 3 to 7. Referring to FIG. 2, the collet 20 is tubular and has abore 22 which is coaxial with the main bore 12 when the collet 20 isinserted within the main bore 12. Each collet 20 has two sets offlexible fingers and a protrusion 24 is provided at the end of eachfinger. Each finger is bendable, when a transverse force is applied tothe protrusion 24, between a first position in which the protrusion 24is outwith the bore 22 of the collet 20 and a second position in whichthe protrusion 24 is within the bore 22. When the collet 20 is insertedwithin the main bore 12, each protrusion 24 is at the first positionwhen engaged with a recess 16 and at the second position when theprotrusion 24 is not engaged with a recess 16.

The first set of fingers 26 and the second set of fingers 28 arelongitudinally spaced apart by a predetermined distance. This distanceis configured so that, when the fingers 26 of the first set are engagedwith a recess 16, the fingers 28 of the second set are not engaged witha recess 16, rather they are between two adjacent recesses 16 and so atthe second position.

The collet 20 is adapted such that a dropped object such as a ball 30can pass through the main bore 12 but it will contact the protrusion 24of any fingers which are at the second position. FIGS. 4 to 7 show aball 30, dropped from the surface and travelling in direction 100,passing through the collet 20.

As shown in FIG. 4, each protrusion 24 of the second set of fingers 28is engaged with a recess 16 and so are unbent and at the first position.However, the protrusions 24 of the first set of fingers 26 are engagedwith a recess 16 and so are bent inwards to the second position. Itshould be noted that the collet 20 could be configured such that thefirst set of fingers 26 are at the first position and the second set offingers 28 are at the second position.

As shown in FIG. 5, the ball 30 contacts the protrusions 24 of the firstset of fingers 26 since they are within the bore 22. One or both of theimpact force from the ball 30 and fluid pressure upstream of the ball 30then causes the collet 20 to be linearly moved in the travel direction100. This causes the second set of fingers 28 to disengage from therecess 16 and linearly move to a location between this recess 16 and thenext recess 16. These fingers 28 are now at the second position. At thesame time, the first set of fingers 26 move forward to engage with thenext recess 16 causing the fingers 26 to unbend to the first position.The protrusions 24 and recesses 16 are suitably profiled to allow theprotrusion 24 to disengage from the recess 16 when a sufficient linearforce is applied.

FIG. 6 shows the fingers in their new positions. Also, with the firstset of fingers 26 at the first position, the ball 30 is free to continueits travel until it meets the second set of fingers 28. Since these arenow at the second position, the ball 30 is stopped at this location.

Again, the impact force from the ball 30 and/or fluid pressure upstreamof the ball 30 causes the collet 20 to be linearly moved in the traveldirection 100. This causes the first set of fingers 26 to disengage fromthe recess 14 and linearly move to a location between this recess 14 andthe next recess 14. These fingers 26 are now at the second position. Atthe same time, the second set of fingers 28 move forward to engage withthe next recess 14 causing the fingers 28 to unbend to the firstposition.

FIG. 7 shows the fingers in their new positions. It should be noted thatthese positions are the same as their original positions before the ball30 approached the collet 20. With the second set of fingers 28 at thefirst position, the ball 30 is free to continue its travel along thewell bore, exiting this tool 10. The ball 30 will continue to travelthrough a tubular to the next tool 10 where it will drive forward thecollet 20 associated with the tool 10 and so on until the last tool isreached.

Therefore, the overall effect of the ball 30 passing through the tools10 is that the associated collet 20 is linearly moved forward one recess16. Any subsequently dropped balls 30 would have the same effect. Thecollet 20 is therefore linearly moved in a stepwise sequence, moving onerecess 16 every time a ball 30 is dropped.

Each tool 10 includes a sleeve 40, as shown in FIGS. 1 and 3. The sleeve40 includes a number of apertures 42. In its normal position, the sleeve40 is connected to the main bore 12 by a connecting member or shear pinand, at this position, the apertures 42 are longitudinally spaced fromthe transverse ports 14. Therefore, the sleeve 40 blocks the transverseports 14 to fluid within the main bore 12. FIG. 2 shows this normalposition with the transverse ports 14 blocked. Seals are provided toprevent leakage of fluid from the main bore 12 to the transverse ports14.

As shown in FIG. 3, a second collet 50 is provided within the main bore12 just downstream of the sleeve 40. With the sleeve 40 in its normalposition, the protrusion of the fingers 52 of the second collet 50 areengaged with second recesses 18 provided at the main bore 12. Therefore,the second collet 50 is unaffected by any dropped balls 30 passingthrough the tool 10.

When a predetermined number of balls 30 have been dropped for theparticular tool 10, the collet 20 will have been moved to reach andcontact the sleeve 40 and this is termed the actuation site. Furtherlinear movement of the collet 20 applies a longitudinal force on thesleeve 40 to linearly move the sleeve 40 when the force is great enoughto cause shearing of the shear pin. This movement of the sleeve 40causes alignment of the apertures 42 of the sleeve 40 and the transverseports 14 so that there is fluid communication between the main bore 12and the transverse ports 14. The movement also causes the sleeve 40 toact upon and linearly move the second collet 50 such that theprotrusions of the fingers 52 of the second collet 50 disengage withsecond recesses 18. A dropped ball 30 will stop at these protrusions andblock the main bore 12.

Therefore, the main bore 12 is now blocked and the transverse ports 14are open. The tool 10 has been actuated and fluid travelling in the wellbore in direction 100 will be diverted out of the tool 10 via thetransverse ports 14.

The apparatus can be arranged so that the collet 20 is located withinthe main bore 12 of a particular tool 10 at a predetermined number ofrecesses 16 from the actuation site. The tools 10 can be arranged sothat this predetermined number of recesses 16 varies for each tool 10depending on its proximity to the surface. The tool 10 furthest from thesurface can involve only one recess 16, while the tool 10 nearest thesurface could have the greatest number of recesses 16, such as fifty.The tools 10 with a collet 20 which is a smaller number of recesses 16from the sleeve 40 will actuate first. The tools 10 will thereforesequentially actuate in the order of furthest away to nearest.

Therefore, each tool 10 is provided with indexing means which is adaptedto register receipt of an actuating signal (the dropped ball 30) and tocause actuation of the tool 10 when a predetermined number of actuatingsignals has been received. At least two of the tools 10 is actuated whena different predetermined number of actuating signals has been receivedand so the downhole tools 10 are sequentially actuatable.

Also, the predetermined number of recesses 14 for each tool 10corresponds to the predetermined number of actuating signals. This maybe an identically correspondence, or the predetermined number ofrecesses could equal, say, the predetermined number of actuating signalsminus one. This would be the case if the collet 20 is moved, say, fourrecesses 14 to move the sleeve and a fifth ball 30 is used to block themain bore 12 (rather than the fourth ball 30 moving the sleeve beforebeing caught by the second collet 50).

The present invention allows each tool 10 to have a valve seat of thesame size and to have a main bore of the same size which issubstantially equivalent to the bore through the tubular. Each ball 30dropped is also the same size. It should also be noted that themechanical counting device of the present invention is non-electrical,non-electronic and non-magnetic. Rather, it is a fully mechanicalapparatus.

FIG. 8 shows an alternative mechanical counting device which is a dogassembly 60 that may be used with the tool 10. In this embodiment, twosets of dogs 62 are provided, rather than the fingers of the collet 20.Each set of dogs 62 are equispaced around the tubular body 64 of the dogassembly 60. As before, the dogs 62 are engagable with recesses 16 ofthe tool 10.

Each dog 62 comprises a block of material, such as steel which isprovided within an aperture 66 of the tubular body 84. Each dog 62 isthicker than the thickness of the tubular body 64 and is movable betweena first position in which the under surface of the dog 62 is flush withthe inner surface of the tubular body 64 (and so does not protrude intothe bore 68 of the tubular body 64) and a second position in which thedog 62 protrudes into the bore 22. FIG. 8 (b) shows both positions. Eachdog 62 includes two wings 70 to prevent the dog 62 from escaping theaperture 66 and falling into the bore 68.

A dropped ball 30 will contact the dogs 62 of the first set since theyare within the bore 68. The dog assembly 60 will then be linearly movedin the travel direction 100 which causes the dogs 62 of the second setto disengage from the recess 16 and linearly move to the secondposition. At the same time, the dog 62 of the first set will moveforward to the first position. The ball 30 is now free to continueforward until it meets the dog 62 of the second set since they are nowat the second position.

The dog assembly 60 is then linearly moved as the ball 30 acts upon thedogs 62 of the second set. This causes the dogs 62 of the first set todisengage from the recess 16 and linearly move to the second position.At the same time, the dogs 62 of the second set move forward to engagewith the next recess 16. The ball 30 is now free to continue its travelalong the well bore, exiting this tool 10.

Whilst specific embodiments of the present invention have been describedabove, it will be appreciated that departures from the describedembodiments may still fall within the scope of the present invention.

1. A downhole actuating apparatus comprising: a plurality of downholetools sequentially arrangable within and along a tubular positionedwithin a well bore, each tool defining a main bore having a diametersubstantially equal to the diameter of the bore through the tubular, andeach tool being actuatable to open one or more fluid ports which aretransverse to the main bore; and an indexer provided at each tool andadapted to register receipt of an actuating signal, and to causeactuation of the tool when a predetermined number of actuating signalshas been received, wherein the indexer of at least two of the tools isadapted to cause actuation when a different predetermined number ofactuating signals has been received such that the downhole tools aresequentially actuatable.
 2. An apparatus as claimed in claim 1, whereinthe predetermined number of actuating signals of each tool is configuredto increase from the tool furthest from the surface to the tool nearestthe surface when the tools are sequentially arranged along the wellbore.
 3. An apparatus as claimed in claim 1, wherein the indexer isadapted to register the presence of an object transported within thetubular, which thus provides the actuating signal.
 4. An apparatus asclaimed in claim 3, wherein the indexer is adapted to register thenumber of objects transported within the tubular, and wherein the toolis actuated when the presence of a predetermined number of droppedobjects has been registered.
 5. An apparatus as claimed in claim 1,wherein each tool has a valve seat located within the main bore, eachvalve seat being of substantially the same size.
 6. An apparatus asclaimed in claim 4, wherein the indexer is adapted, when thepredetermined number of actuating signals has been received, to causethe object to stop at the tool, thus blocking the main bore at the tool.7. An apparatus as claimed in claim 6, wherein the indexer is adapted toreduce the size of the valve seat so as to cause the object to stop atthe tool.
 8. An apparatus as claimed in claim 1, wherein the indexer isa linear indexer.
 9. An apparatus as claimed in claim 1, wherein theindexer comprises a movable device adapted to move in response toreceiving an actuating signal.
 10. An apparatus as claimed in claim 9,wherein the movable device is adapted to linearly progress along themain bore in response to receiving an actuating signal.
 11. An apparatusas claimed in claim 10, wherein the movable device is adapted tolinearly progress towards an actuation site and, upon reaching theactuation site, to cause the actuation of the tool.
 12. An apparatus asclaimed in claim 11, wherein the movable device is adapted to linearlyprogress in a number of discrete steps to the actuation site, the numberof discrete steps corresponding to the predetermined number of actuatingsignals of the tool.
 13. An apparatus as claimed in claim 12, whereinthe movable device comprises a collet member having a number of fingersand a protrusion provided at the end of each finger.
 14. An apparatus asclaimed in claim 13, wherein the collet member comprises a tubularmember having a bore which is sized such that the object may passthrough the tubular member.
 15. An apparatus as claimed in claim 14,wherein each finger is movable between a first position in which theprotrusion is outside the bore of the tubular member and a secondposition in which the protrusion is within the bore of the tubularmember and contactable by the object.
 16. An apparatus as claimed inclaim 15, wherein each finger is bendable between the first and secondpositions.
 17. An apparatus as claimed in claim 1, wherein the main boreof each tool defines a plurality of recesses, and wherein the colletmember is locatable within the main bore such that the protrusion of oneor more fingers is engaged with a recess when the finger is at the firstposition and not engaged with a recess when the finger is at the secondposition.
 18. An apparatus as claimed in claim 17, wherein the colletmember comprises a first set of fingers and a second set of fingerswhich is longitudinally spaced from the first set, and wherein thecollet member and the recesses are configured such that, when thefingers of the first set are engaged with a recess, the fingers of thesecond set are not engaged with a recess.
 19. An apparatus as claimed inclaim 18, wherein the collet member is adapted such that the droppedobject passing through the main bore contacts the protrusion of the oneor more fingers which are at the second position such that the colletmember is linearly moved in the direct of travel of the object until theprotrusion engages with the next recess.
 20. An apparatus as claimed inclaim 19, wherein the collet member is adapted such that the linearmovement of the collet member causes the protrusion of the one or morefingers which are at the first position to disengage from the recess andmove to the second position.
 21. An apparatus as claimed in claim 1,including a sleeve member provided within the main bore adapted to blockthe transverse ports.
 22. An apparatus as claimed in claim 21, whereinthe sleeve member includes at least one connecting member for connectingthe sleeve member to the main bore.
 23. An apparatus as claimed in claim22, wherein the sleeve member includes one or more apertures which arelongitudinally spaced from the transverse port when the sleeve member isconnected to the main bore.
 24. An apparatus as claimed in claim 23,wherein the collet member is adapted to contact and act upon the sleevemember upon reaching the actuation site to move the sleeve member, thuscausing fluid communication between the main bore and the transverseports.
 25. An apparatus as claimed in claim 24, including a secondcollet member provided downstream of the sleeve member, and wherein theapparatus is adapted such that movement of the sleeve member causes thesecond collet member to disengage a recess such that an object isstopped by the second collet member.
 26. An apparatus as claimed inclaim 13, wherein the collet member is located within the main bore ofthe tool at a predetermined number of recesses from the actuation site,the predetermined number of recesses corresponding to the predeterminednumber of actuating signals.
 27. A method of sequentially actuating aplurality of downhole tools which are sequentially arranged within andalong a well bore, the method comprising the steps of: providing indexerat each tool which is adapted to register receipt of an actuatingsignal; causing actuation of the tool when a predetermined number ofactuating signals has been received; configuring at least two of thetools to be actuated when a different predetermined number of actuatingsignals has been received; and sending a number of actuating signals tothe plurality of tools, the number being at least equal to the highestpredetermined number of actuating signals.
 28. A method as claimed inclaim 27, wherein each tool defines a main bore having a diametersubstantially equal to the diameter of the bore through a tubularpositioned within the well bore, and each tool is actuatable to open oneor more fluid ports which are transverse to the main bore.
 29. A methodas claimed in claim 27, including increasing the predetermined number ofactuating signals of each tool from the tool furthest from the surfaceto the tool nearest the surface.
 30. A method as claimed claim 27,including adapting the indexer to register the presence of at least oneobject transported within the tubular, which thus provides the actuatingsignal.
 31. A method as claimed in claim 30, including adapting theindexer to register the presence of a plurality of objects transportedwithin the tubular, each object being substantially the same size.
 32. Amethod as claimed in claim 30, including, when the predetermined numberof actuating signals has been received, causing the object to stop atthe tool, thus blocking the main bore at the tool.
 33. A method asclaimed in claim 27, including linearly moving a movable device towardsan actuation site in response to receiving an actuating signal whereuponthe device causes actuation of the tool.
 34. A method as claimed inclaim 33, including moving the movable device in a number of discretesteps to the actuation site, the number of discrete steps correspondingto the predetermined number of actuating signals of the tool.
 35. Adownhole actuating system comprising: a plurality of downhole toolssequentially arrangable within and along a tubular positioned withinwell bore, each tool defining a main bore having a diametersubstantially equal to the diameter of the bore through the tubular, andeach tool being actuatable to open one or more fluid ports whichtransverse to the main bore; an indexer provided at each tool andadapted to register receipt of an actuating signal, and to causeactuation of the tool when a predetermined number of actuating signalshas been received, wherein the indexer of at least two of the tools isadapted to cause actuation when a different predetermined number ofactuating signals has been received such that the downhole tools aresequentially actuatable; and a plurality of objects adapted to betransported through the tubular, and each of said plurality of objectsprovides the actuating signal.
 36. The system of claim 34 wherein saidplurality of objects comprise a plurality of balls which havesubstantially the same size.
 37. A downhole fracturing tool, comprising:a housing defining a main bore and a fluid port which is transverse tothe main bore; a sleeve actuatable to move from a first configuration inwhich the transverse fluid port is blocked and a second configuration inwhich the transverse fluid port is opened; and an indexer mounted withinthe housing on one axial side of the sleeve and arranged to progresslinearly along the main bore of the housing in a predetermined number ofdiscrete steps of linear movement by passage of a corresponding numberof objects through the indexer to cause said sleeve to move towards itssecond configuration.
 38. The fracturing tool of claim 37, wherein theindexer is arranged such that a final discrete step of linear movementof the indexer causes said sleeve to move towards its secondconfiguration.
 39. The fracturing tool of claim 37, wherein the toolfurther comprises a seat located on an opposite axial side of the sleeveto catch an object which has passed through both the indexer and thesleeve to block the main bore and divert fluid from the main borethrough the transverse port once opened.
 40. The fracturing tool ofclaim 37, wherein each discrete step of linear movement is caused byimpact of a passing object against the indexer in the direction of saidmovement.
 41. A method for fracturing a well, comprising: arranging afracturing tool within a wellbore; delivering a number of objectsthrough an indexer mounted within a main bore of the tool to linearlyprogress the indexer along said main bore in a corresponding number ofdiscrete steps of linear movement to actuate a sleeve located on oneaxial side of the indexer to cause said sleeve to move and unblock afluid port which is transverse to the main bore; and flowing afracturing fluid through the opened transverse fluid port.
 42. A methodas claimed in claim 41, further comprising: delivering an object throughthe indexer to linearly move the indexer one discrete step along themain bore; and delivering at least one further object through theindexer to linearly move the indexer one further discrete step along themain bore to cause the sleeve to move and unblock the one or more fluidports.
 43. A method as claimed in claim 41, further comprising impactingan object against the indexer to move said indexer a discrete step alongthe main bore.
 44. A method as claimed in claim 41, further comprisingblocking the main bore to divert flow from the main bore through theopened transverse fluid ports.
 45. A method as claimed in claim 44,comprising blocking the main bore at a location on a side of the sleeveopposite to that of the indexer.
 46. A method as claimed in claim 44,comprising blocking the main bore with an object which has actuated theindexer to move by a discrete step.
 47. A method as claimed in claim 44,comprising blocking the main bore with an object which has actuated afinal discrete step of the indexer.
 48. A wellbore fracturing system,comprising: a tubing string extending along a wellbore; first and secondfracturing tools arranged along the tubing string, wherein each toolincludes: a housing defining a main bore in communication with thetubing string and a fluid port which is transverse to the main bore; asleeve actuatable to move from a first configuration in which thetransverse fluid port is blocked and a second configuration in which thetransverse fluid port is opened; and an indexer mounted within thehousing on one axial side of the sleeve and arranged to progresslinearly along the main bore of the housing in a predetermined number ofdiscrete steps of linear movement by passage of a corresponding numberof objects through the indexer to cause said sleeve to move towards itssecond configuration, wherein the indexer of the first and second toolsare arranged to actuate the respective sleeves upon passage of adifferent number of objects.
 49. The system of claim 48, wherein thefirst tool is located downhole of the second tool, and the first tool isarranged to receive an object which has passed through the second toolsuch that said object actuates a discrete linear step of the indexer ofeach tool.
 50. The system of claim 48, wherein the first tool is locateddownhole of the second tool, and the indexer of the first tool isarranged to move the associated sleeve of the first tool upon passage ofa lower number of objects than the indexer of the second tool.